The present invention generally relates to methods of producing thin film transistors, and more particularly to a method of producing a thin film transistor wherein a predetermined process is carried out during a production process using polysilicon so as to improve the thin film transistor characteristic.
Recently, there is active development in thin film transistors which are used for driving an image sensor, a liquid crystal display and the like. In addition to improving the thin film transistor characteristic, it is expected that the thin film transistor is applied to various technical fields.
In a metal oxide semiconductor (MOS) thin film transistor which uses polysilicon, a trap state exists at a polysilicon grain boundary due to a dangling bond and a trapping of carriers occurs. Hence, a barrier potential is formed along the grain boundary and a carrier mobility decreases, thereby suffering a problem in that an ON current is small.
As a measure against the above problem, hydrogen atoms are usually introduced to the polysilicon grain boundary so as to correct the disturbance of the crystal lattice. The density of the dangling bond is reduced to reduce the trap state, and the height of the barrier potential is reduced to improve the carrier mobility. For example, the hydrogen atoms are conventionally introduced to the polysilicon grain boundary as described hereunder.
In FIG. 1A, a thin film transistor has an insulator sustrate 1 made of silicon (Si) or the like, a polysilicon active layer 2, a polysilicon diffusion layer 3, a gate oxide layer 4 made of silicon dioxide (SiO.sub.2) or the like, a gate electrode 5, an interlayer insulator layer 6, a wiring layer 7 made of aluminum (Al) or the like, and a passivation layer 8. After this thin film transistor is formed, hydrogen atoms are introduced to the active layer 2 and the diffusion layer 3 by use of a radio frequency (RF) hydrogen discharge plasma chemical vapor deposition (CVD).
In FIG. 1B, those parts which are essentially the same as those corresponding parts in FIG. 1A are designated by the same reference numerals. In this case, after the thin film transistor is formed, hydrogen ions are implanted to the active layer 2 and the diffusion layer 3 followed by an activation at approximately 400.degree. C.
In FIG. 1C, those parts which are essentially the same as those corresponding parts in FIG. 1A are designated by the same reference numerals. In this case, after the thin film transistor is formed, a passivation SiN.sub.x H.sub.y layer is formed and hydrogen atoms are introduced to the active layer 2 and the diffusion layer 3 by a thermal diffusion.
However, the methods described in conjunction with FIGS. 1A through 1C all introduce the hydrogen atoms to the active layer 2 and the diffusion layer 3 in a final process after the thin film transistor is formed. In other words, a process is required in addition to the processes of forming the thin film transistor, and an increase in the number of production steps is unavoidable.
In addition, as may be seen from FIGS. 1A through 1C, the gate oxide layer 4, the gate electrode 5 and the interlayer insulator layer 6 are stacked at the active layer 2 and the diffusion layer 3 where the hydrogen atoms are to be introduced. For this reason, an appropriate process conditions must be satisfied in order that the hydrogen atoms reach the active layer 2 and the diffusion layer 3. That is, a high output is required of the RF hydrogen discharge plasma CVD in the case shown in FIG. 1A, a high implantation energy and high temperature for the activation are required in the case shown in FIG. 1B, and a high hydrogen density within the passivation SiN.sub.x H.sub.y layer and a high temperature for the thermal diffusion are required in the case shown in FIG. 1C. However, these process conditions must be set under a restriction that no damage is made to the thin film transistor. Therefore, the conventional methods of forming the thin film transistors are not efficient from this point of view.